Regulation of Body Temperature

Outline the mechanisms for heat transfer between the body and its environment.

Define the thermoneutral zone, and describe the mechanisms by which normal body temperature is maintained.

Regulation of body temperature is done by balancing heat loss and heat production, predominantly through behavioural mechanisms and skin

The body is able to maintain a relatively constant core temperature under a wide range of environmental conditions

The thermoneutral zone is the range across which the basal rate heat production (and oxygen consumption) is balanced by the rate of heat loss

For an adult it is typically 27-31°C

In neonates it is higher, typically 32-34°C.

Principles

Net flux of heat is determined by the balance of metabolic heat production and the contribution of four mechanisms of heat loss:

Radiation

Conduction

Convection

Evaporation

Radiative

Radiative heat exchange:

Describes the loss of heat through EMR by all objects above 0°K

Radiative heat loss is proportional to temperature

Radiative heat loss does not require a transfer medium

Makes up ~45% of heat loss under thermoneutral conditions.

Depends on the temperature differential between an individual and their environmen

A cold environment (e.g. operating theatre) causes a large radiant heat lossThe heat loss from the patient is greater than the heat gain from the surrounding environment.

Conduction

Conduction is the transfer of heat (as kinetic energy) by direct contact from a higher temperature object to the lower temperature one. Conduction:

Requires physical contact between bodies to conduct heat

Solids conduct heat better than gases

There is no conduction in a vacuum

Heat loss via conduction is minimal in air but is a major cause of heat loss in immersion

As arteries and veins typically run next to each other, arterial heat tends to be transferred to the (cooler) veins, limiting further heat lossThis is similar to counter-current exchange in the kidney.

As fat is a poorer conductor of heat than muscle, increased body fat will slow heat loss by conduction

Convection

Convection is loss of heat by conduction by a moving object. Convection is:

The predominant mechanism of heat loss in the naked humanEffects are greater effects at higher wind speeds.

Evaporation

Evaporative losses describe the loss of heat energy due to the latent heat of vaporisation of water. Evaporation of 100ml of water will reduce body temperature by ~1°C.

Temperature Sensation and Regulation

Central sensation

Central temperature sensors exist in the:

Abdomial viscera

Spinal cord

HypothalamusAnterior hypothalamus is the most important central thermoreceptor, and responds to both increased and decreased temperatures by altering their rate of depolarisation, eliciting an array of neuronal and hormonal responses.

Brainstem

The interthreshold range is the range of core temperatures not triggering a response.

Normal is 0.2 to 0.4°C.

Widens under anaesthesia to ~4°C

Peripheral sensation

Peripheral temperature sensors are:

Free nerve endings

Extremely sensitiveAlter their rates of firing by orders of magnitude in response to temperature change.

Divided into:

Cold receptorsLie beneath the epidermis, and are excited by cooling (inhibited by warming), active from 10-40°C, with a static maxima at 25°C.

Warm receptorsLie deep to the dermis, are excited by warming (and inhibited by cooling), active from 30-50°C, with a static maxima at 44°C.

Regulation

Temperature sensation runs from cutaneous receptors via the spinothalamic tracts and medulla to the hypothalamus. Cortical input is received via the thalamocortical relay, whilst primitive responses are effected via the midbrain.

Vascular changes are the least metabolically costly and can result in dramatic increases (up to 60% of cardiac output) in skin blood flow

When environmental temperature exceeds body temperature, conduction and convection result in heat gain - evaporative cooling via sweating is the only way to reduce body temperature

Efficacy of sweating is related to relative humidity

Piloerection (hair standing on end) traps a layer of warm air close to the body to act as an insulatorThis is of more importance in other primates than in man, as they have enough body hair to make it effective.

Increasing basal metabolic rate and 'waste' heat production is essential to maintain temperature in cold environments. This can be through:

A vital mechanism for heat production in the neonate (they have an immature shivering response), and forms ~5% of neonatal mass

Located in:

Neck

Supraclavicular

Interscapula

Suprarenal

Sympathetically innervatedContains large numbers of β3 receptors

Effect of Anaesthesia

General anaesthesia causes a 1-3°C drop in core body temperature, which occurs in three phases:

Rapid reductionCore temperature falls by 1-1.5°C in the first 30 minutes.

Predominantly due to vasodilation, which is due to:

Reduction in SVR, with generalised vasodilation and increased skin blood flowHeat redistribution is the major initial factor (rather than heat loss), as vasodilation leads to increased heat content of peripheries.